U.S. patent application number 10/748668 was filed with the patent office on 2004-11-25 for filter circuit.
Invention is credited to Huang, Jui-Cheng, Lee, Chao-Cheng, Tsai, Jui-Yuan, Wang, Wen-Chi.
Application Number | 20040232977 10/748668 |
Document ID | / |
Family ID | 32734564 |
Filed Date | 2004-11-25 |
United States Patent
Application |
20040232977 |
Kind Code |
A1 |
Lee, Chao-Cheng ; et
al. |
November 25, 2004 |
Filter circuit
Abstract
A filter circuit of the present invention provides a
transconductance device for outputting a current signal according
to an input voltage and a feedback voltage; a transresistance
device coupled to the transconductance device for outputting a
output voltage according to the current signal; and a feedback
device coupled between the transconductance device and the
transresistance device for outputting the feedback voltage
according to the output voltage. The transresistance device is
coupled to the transconductance device via a resistor network
comprising a plurality of stages connected serially, wherein each
stage of the resistor network comprises: an input node; an output
node; a first resistor coupled between the input node and the
ground; and a second resistor coupled between the input node and
the output node.
Inventors: |
Lee, Chao-Cheng; (HsinChu,
TW) ; Huang, Jui-Cheng; (Hsinchu City, TW) ;
Tsai, Jui-Yuan; (Tainan City, TW) ; Wang,
Wen-Chi; (Siluo Township, TW) |
Correspondence
Address: |
RABIN & Berdo, PC
1101 14TH STREET, NW
SUITE 500
WASHINGTON
DC
20005
US
|
Family ID: |
32734564 |
Appl. No.: |
10/748668 |
Filed: |
December 31, 2003 |
Current U.S.
Class: |
327/552 |
Current CPC
Class: |
H03H 11/1217 20130101;
H03H 11/0466 20130101; H03H 7/38 20130101 |
Class at
Publication: |
327/552 |
International
Class: |
H03B 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 10, 2003 |
TW |
92100494 |
Claims
What is claimed is:
1. A filter circuit, comprising: a transconductance device for
outputting a current signal according to an input voltage and a
feedback voltage; a transresistance device coupled to the
transconductance device for outputting a output voltage according
to the current signal, wherein the transresistance device
comprises: a first capacitor; a resistor network coupled to the
capacitor and the transconductance device comprising a plurality of
stages connected serially, wherein each stage of the resistor
network comprises: an input node; an output node; a first resistor
coupled between the input node and the ground; and a second
resistor coupled between the input node and the output node;
wherein a time constant of the filter circuit is determined by the
first capacitor and the resistor network; and a feedback device
coupled between the transconductance device and the transresistance
device for outputting the feedback voltage according to the output
voltage.
2. The filter circuit as claimed in claim 1, wherein the
transconductance device comprises: a first operational amplifier
having a first non-converting input terminal coupled to a ground, a
first converting input terminal and a first output terminal to
output the current signal; a first resistor coupled to the first
output terminal and the first converting input terminal; and a
second resistor coupled to the first converting input terminal for
receiving the input voltage.
3. The filter circuit as claimed in claim 1, wherein the
transresistance device comprises: a second operational amplifier
having a second non-converting input terminal coupled to a ground,
a second converting input terminal and a second output terminal to
output the output voltage; the first capacitor coupled to the
second output terminal and the second converting input terminal;
and the resistor network coupled to the second converting input
terminal for receiving the current signal.
4. The filter circuit as claimed in claim 3, wherein the resistance
of the first resistor is two times larger than the resistance of
the second resistor.
5. The amplifier circuit as claimed in claim 4, wherein the
equivalent resistance of the resistor network is 2.sup.n.times.R,
wherein the resistor network includes n stages and the resistance
of the second resistor is R.
6. The amplifier circuit as claimed in claim 3, wherein each of the
first resistor and the second resistor is implemented by a MOS
transistor.
7. The filter circuit as claimed in claim 1, wherein the feedback
device comprises: a third operational amplifier having a third
non-converting input terminal coupled to a ground, a third
converting input terminal and a third output terminal to output the
output voltage; a third resistor coupled to the third output
terminal and the third converting input terminal; and a fourth
resistor coupled to the third output terminal for outputting the
feedback voltage.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is related in its subject matter to
that of the applicants' copending U.S. patent application entitled
AMPLIFIER CIRCUIT, filed concurrently herewith on Dec. 31, 2003,
which is commonly owned by the assignee of the present application,
and the disclosure of which is hereby incorporated by reference.
The present application also incorporates by reference the
disclosure of applicants' prior corresponding Taiwan Application
No. 92100495, which was filed Jan. 10, 2003, the foreign priority
benefit of which is claimed herein.
BACKGROUND OF THE INVENTION
[0002] The present invention relates in general to a filter
circuit. In particular, the present invention relates to a low-pass
filter circuit with variable low cut-off frequency.
[0003] Description of the Related Art
[0004] Filters are common elements in communication systems.
Filters adjust the waveform of signal, suppress harmonic
interference, and decrease the noise in the communication system.
Recently, smaller size and higher quality filters are required in
mobile communication systems.
[0005] FIG. 1 is a circuit showing the conventional low-pass
filter. In FIG. 1, the cut-off frequency is 1 1 R1 C1 .
[0006] When the cut-off frequency is set as 10 Hz, the product of
the resistance of the resistor R1 and the capacitance of the
capacitor C1 must be 2 1 2 10 .
[0007] A reasonable capacitance of a capacitor made by common
semiconductor process, however, is 10 Pf. Thus, when the
capacitance of a capacitor C1 is 10 Pf, the resistance of the
resistor R1 must be 1592 Meg. It is costly, however, to fabricate a
resistor with resistance of 1592 Meg, which is an unreasonable
value. The area requirement of the common semiconductor process to
form a resistor with the resistance of 1592 Meg must be 1262 u-1262
um.sup.2, which is unreasonable large to the modern IC circuit
device. Thus, it is difficult to form a resistor having a very
large resistance. Thus, the cut-off frequency of the conventional
filter is limited by the resistance and the capacitance of the
semiconductor device, thus conventional filter quality suffers.
SUMMARY OF THE INVENTION
[0008] The object of the present invention is thus to provide a
low-pass filter circuit using a resistor network structure
significantly with large equivalent resistance without occupying a
large IC area.
[0009] To achieve the above-mentioned object, the filter circuit of
the present invention provides a transconductance device for
outputting a current signal according to an input voltage and a
feedback voltage; a transresistance device coupled to the
transconductance device for outputting a output voltage according
to the current signal; and a feedback device coupled between the
transconductance device and the transresistance device for
outputting the feedback voltage according to the output voltage.
The transresistance device is coupled to the transconductance
device via a resistor network comprising a plurality of stages
connected serially, wherein each stage of the resistor network
comprises: an input node; an output node; a first resistor coupled
between the input node and the ground; and a second resistor
coupled between the input node and the output node.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The present invention will become more fully understood from
the detailed description given hereinbelow and the accompanying
drawings, given by way of illustration only and thus not intended
to be limitative of the present invention.
[0011] FIG. 1 is a circuit showing the conventional low-pass
filter.
[0012] FIG. 2 is a circuit of a resistor ladder comprising five
stages.
[0013] FIG. 3 is a circuit showing the low-pass filter circuit
according to one embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0014] The low-pass filter circuit according to one embodiment of
the present invention implements an additional negative feedback
path and a resistor network to decrease the required resistance of
the filter circuit. The circuit structure of the resistor network
is described in the following.
[0015] FIG. 2 is a circuit of a resistor network comprising five
stages. The resistances of the resistors can be set to any
combination. Here, the resistance of the resistors R10, R11, R13,
R15, R17 and R19 is set as the twice that of the resistors R12,
R14, R16 and R18. The equivalent circuit of the resistor network is
the resistance of the parallel connection of the resistors R10 and
R11 is 1R, is then connected in series to the resistor R12, thus
the equivalent resistance is 2R. Next, the equivalent resistor is
connected in parallel to the R13 and so on. Thus, the resistance of
each current path at the nodes 20, 22, 24, 26 and 28 is 2R.
Therefore, when the current I is input to the input terminal Vi1,
the current value of the current is halved when passing through the
nodes 20, 22, 24, 26 and 28. FIG. 2 also shows the value of the
current on each resistor. Because the circuit structure is a
resistor network having five stages, the value of the current
output from the output terminal Vol is I/2.sup.5. In addition, the
output current decreases when the stage number of the resistor
ladder increases.
[0016] FIG. 3 is a circuit showing the low-pass filter circuit
according to one embodiment of the present invention. The low-pass
filter circuit according the embodiment of the present invention
implements an additional negative feedback path and a resistor
network to obtain a filter with excellent low-pass quality.
[0017] The low-pass filter circuit according to the embodiment of
the present invention comprises an adder 30 and an integrator 32.
In addition, a feedback circuit 34 is connected between the input
terminal of the adder 30 and the output terminal of the integrator
32.
[0018] The input terminal of the adder 30 receives input voltage Vi
and a feedback voltage Vf, and outputs an output current Io
according to the sum of the voltage level of the input voltage Vi
and the feedback voltage Vf. The adder 30 comprises an operational
amplifier OP1 having a grounded non-converting input terminal, a
converting input terminal and an output terminal to output the
output current Io. The resistor Rin2 is connected between the
output terminal and the converting input terminal of the
operational amplifier OP1. Here, the value of the output current Io
is the sum of the current passing through the resistors Rin1 and
Rin3.
[0019] The integrator 32 is coupled to the adder 30 to output an
output voltage Vo. The integrator 32 comprises a resistive network
31, a capacitor 33 and an operational amplifier OP2 having a
grounded non-converting input terminal, a converting input terminal
and an output terminal. Here, the capacitor 33 is coupled between
the output terminal and the converting input terminal of the
operational amplifier OP2.
[0020] In addition, the resistive network 31 is composed of a
plurality of stages, wherein the circuit of the resistor network
has been shown in FIG. 2. Each stage of the resistor ladder (21,
23, 25, 27, and 29) includes a first current path and a second
current path, which are connected to the node of the stage. The
first-stage resistor network 21 is connected to the adder 30, and
the last-stage resistor network 29 is connected to the converting
input terminal of the operational amplifier OP2. The first current
path of each stage is connected to the node of the next-stage
resistor ladder, and the second current paths of the resistor
network are all grounded.
[0021] The feedback circuit 34 is coupled between the output
terminal Vo and the converting input terminal of the operational
amplifier OP1 to transfer the output signal of the integrator
circuit 32 to the feedback signal Vf. Here, the feedback signal Vf
is inverted to the output signal of the integrator circuit 32. The
feedback circuit 34 comprises an operational amplifier OP3 having a
grounded non-converting input terminal, a converting input terminal
coupled to the output terminal Vo and a output terminal coupled to
the converting input terminal of the operational amplifier OP1 to
output the feedback signal Vf. The resistor R11 is coupled between
the output terminal of the operational amplifier OP2 and the
converting input terminal of the operational amplifier OP3, and the
resistor R12 is coupled between the output terminal and the
converting input terminal of the operational amplifier OP3.
[0022] If the resistance of the resistors R21 and R22 are the same,
the operational amplifier OP3 generates the reverse voltage of the
output voltage Vo. If, however, the resistance of resistors R21 and
R22 can be adjusted according to feedback to achieve an appropriate
feedback value. The appropriate feedback value is added to the
input voltage Vi, combining the resistor network and the integrator
32, thus a low-pass filter is obtained, which has a cut-off
frequency 1/(Req.times.C1). Here, Req represents the equivalent
resistance of the resistor network. In addition, the integrator 32
according to the embodiment of the present invention implements the
resistor network as the resistive load 31, so the equivalent
resistance Req of the resistive load 31 is R.times.2.sup.N. Using a
16-stage resistor network as an example, the unit resistance is
0.024 Meg. In addition, the total resistance is only 1.176 Meg.
Compared with the conventional low-pass filter circuit, the
low-pass filter circuit of the present invention achieves the same
cut-off frequency by using 1/1353 resistance of the conventional
low-pass filter circuit.
[0023] In addition, the feedback circuit 34 and the adder 30 can be
replaced with a subtractor. In the present invention, the
proportional of the resistance on the first current path and the
second current path isn't limited on 1:2, actually, can be any
other value, for example, 1:3 or 3:2. In other words, a larger
resistance is obtained by using the resistor network with a
plurality of stages.
[0024] Accordingly, the high equivalent resistance of the resistor
network decreases the required resistance of the low-pass filter.
Thus, higher resistance is achieved in the semiconductor device.
Therefore, an ideal low cut-off frequency of the low-pass filter
according to the embodiment of the present invention is achieved
and the filtering effect is improved.
[0025] It should be noted that the resistor network disclosed in
the embodiments of the present invention is suitable to be
implemented inside of the IC device such that the resistor network
can be with large resistance without occupying a large area. In
addition, each resistor of the resistor network can be implemented
by the MOS transistor. The resistance of each resistor and/or the
number of the stages of the resistor network can be determined
through controlling the gate voltage of the corresponding MOS
transistors.
[0026] The foregoing description of the preferred embodiments of
this invention has been presented for purposes of illustration and
description. Obvious modifications or variations are possible in
light of the above teaching. The embodiments were chosen and
described to provide the best illustration of the principles of
this invention and its practical application to thereby enable
those skilled in the art to utilize the invention in various
embodiments and with various modifications as are suited to the
particular use contemplated. All such modifications and variations
are within the scope of the present invention as determined by the
appended claims when interpreted in accordance with the breadth to
which they are fairly, legally, and equitably entitled.
* * * * *